The problem can be corrected by upgrading the affected package to version 1.0.6-0ubuntu0.1. After performing a standard system upgrade you need to restart Firefox to effect the necessary changes.

Details follow:

USN-149-1 fixed several vulnerabilities in the Firefox web browser. Unfortunately that update introduced a lot of regressions, especially when using extensions, so another update is necessary. The new packages ship Firefox version 1.0.6 which should now work well with most extensions (one known exception is the package "mozilla-tabextensions").